1. Field of the Invention
The present invention relates to a technique of controlling a synchronous motor in a sensor-less manner, or more specifically to a technique of controlling a synchronous motor that is rotated at a high speed.
2. Description of the Related Art
In a synchronous motor that rotates a rotor through an interaction between a magnetic field occurring when multi-phase alternating currents are flown through windings and a magnetic field produced by a permanent magnet, in order to obtain a desired rotational torque, it is required to control the multi-phase alternating currents according to the electrical angle or the electrical position of the rotor. The electrical angle may be detected with a sensor, such as a Hall element. It is, however, desirable to detect the electrical angle and control operation of the synchronous motor in a sensor-less manner, with the view to assuring the reliability of a control apparatus of the synchronous motor.
In the case where the motor is driven at a high speed (hereinafter simply referred to as high-speed rotation), one proposed method detects an error of an electrical angle in a sensor-less manner using voltage equations (1) and (2) given below and thereby controls the motor: EQU Vd=R.multidot.Id+p(Ld.multidot.Id) -.omega..multidot.Lq.multidot.Iq(1) EQU Vq=R.multidot.Iq+p(Lq.multidot.Iq) -.omega..multidot.Ld.multidot.Id+.omega..phi. (2)
where V denotes voltages applied to the motor, I electric currents flowing through the windings of the motor, and L inductances of the windings. The subscripts d and q attached to V, I, and L show that the values relate to the d-axis direction and the q-axis direction of the motor. R denotes the coil resistance of the motor, .omega. the rotational angular velocity of the motor, and .omega. the number of flux linkages. Among these arithmetic elements, the coil resistance of the motor R, the inductances L, and the number of flux linkages .omega. are intrinsic to the motor and are thereby referred to as the motor constants. The time derivative operator p is defined as: EQU p(Ld.multidot.Id)=d(Ld.multidot.Id)dt
The d-axis and the q-axis are described briefly with the drawing of FIG. 4. A permanent magnets-type three-phase synchronous motor is expressed by an equivalent circuit shown in FIG. 4. In this equivalent circuit, the direction that passes through the center of rotation of a rotor and goes along a magnetic field formed by a permanent magnet is generally referred to as the d-axis, whereas the direction that is electrically perpendicular to the d-axis in the rotational plane of the rotor is generally referred to as the q-axis. In the equivalent circuit of FIG. 4, the angle of the U-phase and the d-axis corresponds to an electrical angle .theta. of the motor.
The following describes the idea of the motor control technique using the voltage equations (1) and (2). The voltage equations (1) and (2) are always valid with respect to the d-axis and the q-axis, but the accurate value of the electrical angle is unknown in the case of the sensor-less control of the motor. The motor control apparatus accordingly solves the voltage equations (1) and (2) with an estimated electrical angle (corresponding to .theta.c in FIG. 4). Errors of the arithmetic operations accordingly exist corresponding to an angular error (.DELTA..theta. in FIG. 4) between the estimated electrical angle .theta.c and the true electrical angle .theta.. It is possible, on the contrary, to determine the error angle based on the errors of the arithmetic operations. Correction of the estimated electrical angle .theta.c with the error angle thus enables the error of the electrical angle to be kept within a specific range and thereby assures adequate control of the motor.
The proposed motor control method corrects the electrical angle and controls the motor in the sensor-less manner in the case where the permanent magnets-type motor is driven at a relatively high speed. In the synchronous motor in this driving state, however, when the torque to be output from the motor exceeds a predetermined torque level depending upon the rating of the motor, this method can not adequately control the motor.
One possible countermeasure to solve the above problem designs the motor to have a sufficiently marginal rating relative to the torque to be output. This, however, undesirably increases the size and the weight of the motor. Especially when the motor is mounted on an electric vehicle, the size and weight reduction of the motor is highly required because of the limited space and weight.